The trend is that, in the future, electronic systems that combine new energy storage technologies with electrical drive technology will be used increasingly both in stationary applications, such as wind power installations or solar installations, and in vehicles, such as hybrid or electric vehicles.
The supply of polyphase current to an electric machine is usually accomplished by a converter in the form of a pulse-controlled inverter. For this purpose, a DC voltage provided by a DC voltage intermediate circuit can be converted into a polyphase AC voltage, for example a three-phase AC voltage. In this case, the DC voltage intermediate circuit is powered by a string of battery modules connected up in series. In order to be able to meet the demands on power and energy for a respective application, a plurality of battery modules are frequently connected in series in a traction battery.
A series circuit comprising a plurality of battery modules is associated with the problem that the entire string fails when a single battery module fails. Such a failure of the energy supply string can result in failure of the entire system. Furthermore, temporarily or permanently occurring power reductions of a single battery module can result in power reductions in the entire energy supply string.
The document U.S. Pat. No. 5,642,275 A1 describes a battery system with an integrated inverter function. Systems of this kind are known by the name of multilevel cascaded inverter or else battery direct inverter (BDI). Such systems comprise DC sources in a plurality of energy storage module strings that can be connected directly to an electric machine or an electric grid. In this case, single-phase or polyphase supply voltages can be generated. The energy storage module strings in this case have a plurality of series-connected energy storage modules, wherein each energy storage module has at least one battery cell and an associated controllable coupling unit which allows the respective energy storage module string to be interrupted or the respectively associated at least one battery cell to be bypassed or the respectively associated at least one battery cell to be switched into the respective energy storage module string, depending on control signals. By suitable actuation of the coupling units, for example using pulse-width modulation, suitable phase signals can also be provided for controlling the phase output voltage, as a result of which it is possible to dispense with a separate pulse-controlled inverter. The pulse-controlled inverter required for controlling the phase output voltage is therefore integrated in the BDI so to speak.
BDIs usually have a higher level of efficiency and a higher level of failsafety in comparison with conventional systems. Failsafety is ensured, inter alia, by the ability for faulty, failed or not fully effective battery cells to be disconnected from the energy supply strings by virtue of suitable bypass actuation of the coupling units. The phase output voltage of an energy storage module string can be varied, and, in particular, set in a stepped manner, by virtue of appropriate actuation of the coupling units. In this case, the step range of the output voltage is obtained from the voltage of a single energy storage module, with the maximum possible phase output voltage being determined by the sum of the voltages of all the energy storage modules in an energy storage module string.
By way of example, documents DE 10 2010 027 857 A1 and DE 10 2010 027 861 A1 disclose battery direct inverters having a plurality of battery module strings which are directly connectable to an electric machine.
In this case, the energy storage module strings have a plurality of series-connected energy storage modules, wherein each energy storage module has at least one battery cell and an associated controllable coupling unit which makes it possible to bypass the respectively associated at least one battery cell or to switch the respectively associated at least one battery cell into the respective energy storage module string, depending on control signals. Optionally, the coupling unit can be configured such that it additionally allows the respectively associated at least one battery cell also to be switched into the respective energy storage module string with reversed polarity or allows the respective energy storage module string to be interrupted.
BDIs usually have a higher level of efficiency and a higher level of failsafety in comparison with conventional systems. Failsafety is ensured, inter alia, by the ability for faulty, failed or not fully effective battery cells to be disconnected from the respective energy supply string by virtue of suitable bypass actuation of the coupling units. The total output voltage of each energy storage module string can be varied, and, in particular, set in a stepped manner, by virtue of appropriate actuation of the coupling units. In this case, the step range of the output voltage is obtained from the voltage of a single energy storage module, with the maximum possible total output voltage being determined by the sum of the voltages of all the energy storage modules in the energy storage module string.
In order to set an output voltage of an energy storage module, the coupling units are actuated in a pulse-width-modulated (PWM) manner. As a result, it is possible to output a desired average value as energy storage module voltage by targeted variation of the switch-on and switch-off times.
When used in electrically driven vehicles, such as electric cars or hybrid vehicles, for example, it is possible for the temperatures of the battery cells of energy storage devices of this type to be very low, for example during starting of the vehicle in winter. Since battery cells which are usually used have a temperature-dependent internal resistance which increases with decreasing temperature, it is possible that the battery cells cannot provide the full power in low temperatures.
There is therefore a need for a method which can ensure the full effectiveness of the battery cells, even at low temperatures, without elaborate heating elements or a time- and cost-intensive external heating process having to be used.